Overheating

AC pump loads the engine, and causes the old cooling system to react by overheating. Check your entire cooling system out from water pump to radiator, to T-stat... and all hoses. you locate the weak components. Even without the AC ON, watch the closely the temperature is probably still to high. Regards

Also, check for blocked air flow in front of the radiator. Leaves and stuff can accumulate in that area and block the flow of air.

AC use triggers the high speed fan relay. If that relay is bad the fan doesn't speed up to compensate for the ac and the car starts overheating. That relay sometimes goes bad because the cooling fan unit motor is going bad and drawing too much voltage, blowing the relay. So you have to check both. Replacing the $15 fan relay twice is a pain. The design for electric cooling fans is unreliable. The old belt driven fans may not have been as good at cooling, but you never were without some sort of fan moving air through the radiator. In modern cars a relay goes out and suddenly you are overheating fast - without even having lost a drop of coolant due to a hose or radiator leak which used to be the major culprits. So get that cooling fan checked out, replace those relays, check the codes for any sensors malfunctioning and drive that beautiful Lincoln in cool style!